1. Field of the Invention
The present invention relates to a hollow frame member of an aluminum alloy for a vehicle body frame, and particularly, to such a hollow frame member of an aluminum alloy having a collision energy absorbing ability.
2. Description of the Related Art
Vehicle body frames that are formed by various hollow frame members made of an aluminum alloy and coupling members from the demand of a reduction in weight of an automobile are known. In this case, the vehicle body frame is provided, for example, with hollow frame members of an aluminum alloy, which are disposed at left and right side portions of a front portion of a vehicle body so as to extend longitudinally of the body and to exhibit a collision energy absorbing ability by crushing thereof.
To enhance the collision energy absorbing efficiency provided by these hollow frame members, it is required that the hollow frame member be crushed stably from its tip end upon collision of the vehicle, that a peak load on the hollow frame member be small, that a constant load be generated with a small amplitude of load after generation of the peak load, and that a surface receiving a collision load be wide enough to reliably receive the collision load.
However, the conventional hollow frame member is formed to have a large thickness over its entire length as well as a constant cross sectional area, because the aluminum alloy is low in durability from the viewpoint of its characteristic. For this reason, the conventional hollow frame member suffers from the following problems: the crushability thereof is unstable upon collision of the vehicle, and the peak load is relatively large. Moreover, the amplitude of the load after generation of the peak load is large, and a constant load cannot be generated. Further, the surface receiving the collision load is narrow. Therefore, in the vehicle body frame formed using the conventional hollow frame members, it is extremely difficult to sufficiently enhance the collision energy absorbing efficiency.
There is also a known vehicle body frame for an automobile, which constitutes a kick-up portion adjacent left and right front wheels and which is formed by bending an extruded pipe of an aluminum alloy having a circular cross section into a bow shape.
If a collision load is applied to a front end of the hollow frame member due to a frontal collision of a vehicle, a bending deformation is produced at a substantially lengthwise central portion of the hollow frame member so as to increase the bending degree of the frame member in order to absorb such collision load. In this case, a relatively large elongation is produced on an outer peripheral surface portion outside the bent portion, and the breaking of the outer peripheral surface portion occurs, because an aluminum alloy is small in elongation from the viewpoint of its characteristic. Such breaking must be avoided to the utmost, because it reduces the collision energy absorbing efficiency remarkably.